This invention relates to a system for, and method of receiving information (e.g., video and/or data) signals transmitted by a satellite from a plurality of stations each operative in an individual frequency range and for recovering the information represented by the information signals.
Satellites have been in existence for a number of years for receiving signals in space from a plurality of television stations and for transmitting these signals to a subscriber on the ground. Each of the television stations provides signals in an individual range of frequencies. For example, the encoded digital signals from the different television stations may have different data rates in a range between approximately two megabits/second (2 Mb/s) to approximately ninety megabits/second (90 Mb/s).
The satellites receive the signals from the different television stations in the frequency range of approximately 2-90 Mb/s and modulate these signals with a carrier signal having a suitable frequency such as a frequency in the range of approximately nine hundred and fifty megahertz (950 MHz) to approximately twenty one hundred and fifty megahertz (2150 MHz). The satellites then transmit the modulated carrier signals to television receivers on the ground.
The television receivers then convert the carrier signals to signals at an intermediate frequency such as approximately four hundred and eighty megahertz (480 MHz). These intermediate frequency signals are then demodulated at the television receivers and the demodulated signals are processed to recover the data signals from the individual ones of the television stations. The processing of the signals occurs on an analog basis.
It is well recognized that the processing of the signals on an analog basis to recover the data in the data signals is not as precise as would ordinarily be desired. The recovery of such data on a precise basis by analog techniques is especially difficult in view of the fact that the data signals may occur in a range of frequencies as great as approximately two megabits/second (2 Mb/s) to approximately ninety megabits/second (90 Mb/s).
Carrier signals are modulated by information (video and/or data) signals in a particular frequency range. The information signals are oversampled at a first frequency greater than any of the frequencies in the particular frequency range to provide digital signals at a second frequency.
The digital signals are introduced to a carrier recovery loop which provides a feedback to regulate the frequency of the digital signals at the second frequency. The digital signals are also introduced to a symbol recovery loop which provides a feedback to maintain the time for the production of the digital signals in the middle of the information signals. The gain of the digital signals is also regulated in a feedback loop. The digital signals are processed to recover the data in the data signals. By providing digital feedbacks, the information recovered from the digital signals can be quite precise.
In one embodiment, the carrier signals are demodulated to produce baseband inphase and quadrature signals. The inphase and quadrature signals are then oversampled and regulated in the feedback loops as described above.
In a second embodiment, the carrier signals are downconverted to produce intermediate frequency signals which are oversampled to produce the digital signals at the second frequency without producing the inphase and quadrature signals. The oversampled signals are then regulated in the feedback loops as described above.
In a third embodiment, the carrier signals are oversampled without being downconverted or producing the inphase and quadrature signals.